When you file separately you need your spouse's information so the IRS can compare the separate returns. If you don't have her SSN you would have to mail in your return.
If you filed as Married Filing Joint in past years, and you have a copy of the return, her information would be on the Joint return.
You will have to file by mail if you can't get it and are filing Married Filing Separately. You won't be able to e-file without her info. If you have a dependent child living in the home that you provide upkeep for, and you were separated for the last 6 months of 2021, you may be able to claim Head of Household status and in that case, you do not need her SSN to e-file.
When you file separately you need your spouse's information so the IRS can compare the separate returns. If you don't have her SSN you would have to mail in your return.
If you filed as Married Filing Joint in past years, and you have a copy of the return, her information would be on the Joint return.